stat(2): clarify which syscalls modify file timestamps

The list of syscalls that modify st_atim, st_mtim, and st_ctim was quite out
of date and probably not accurate to begin with. Update it, and make it
clear that the list is open-ended.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D18410